Re: [PATCH bpf-next v8 4/4] selftests/bpf: Add tests for bpf_rcu_read_lock()

From: Martin KaFai Lau <martin.lau@linux.dev>
To: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andrii@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	kernel-team@fb.com, Martin KaFai Lau <martin.lau@kernel.org>,
	bpf@vger.kernel.org
Subject: Re: [PATCH bpf-next v8 4/4] selftests/bpf: Add tests for bpf_rcu_read_lock()
Date: Tue, 22 Nov 2022 16:56:51 -0800	[thread overview]
Message-ID: <201c1603-cb3e-7893-c411-e7949ef8e9d3@linux.dev> (raw)
In-Reply-To: <20221122195340.1783247-1-yhs@fb.com>

On 11/22/22 11:53 AM, Yonghong Song wrote:
> +SEC("?fentry.s/" SYS_PREFIX "sys_nanosleep")
> +int task_acquire(void *ctx)
> +{
> +	struct task_struct *task, *real_parent;
> +
> +	task = bpf_get_current_task_btf();
> +	bpf_rcu_read_lock();
> +	real_parent = task->real_parent;
> +	/* acquire a reference which can be used outside rcu read lock region */
> +	real_parent = bpf_task_acquire(real_parent);
Does the bpf_task_acquire() kfunc need a change to do refcount_inc_not_zero() 
and KF_RET_NULL?

Also, some more 'skip' checks in prog_tests/rcu_read_lock.c is needed for gcc. 
This is failing in gcc CI:

https://github.com/kernel-patches/bpf/actions/runs/3527747280/jobs/5917628248#step:6:5624

   ; bpf_rcu_read_lock();
   2: (85) call bpf_rcu_read_lock#26650
   ; real_parent = task->real_parent;
   3: (79) r1 = *(u64 *)(r6 +1416)       ; R1_w=ptr_task_struct(off=0,imm=0) 
R6_w=trusted_ptr_task_struct(off=0,imm=0)
   ; real_parent = bpf_task_acquire(real_parent);
   4: (85) call bpf_task_acquire#26666
   R1 must be referenced or trusted	
   processed 5 insns (limit 1000000) max_states_per_insn 0 total_states 0 
peak_states 0 mark_read 0
   -- END PROG LOAD LOG --
   libbpf: prog 'task_acquire': failed to load: -22

> +	bpf_rcu_read_unlock();
> +	(void)bpf_task_storage_get(&map_a, real_parent, 0, 0);
> +	bpf_task_release(real_parent);
> +	return 0;
> +}
> +
> +SEC("?fentry.s/" SYS_PREFIX "sys_nanosleep")
> +int no_lock(void *ctx)
> +{
> +	struct task_struct *task, *real_parent;
> +
> +	/* no bpf_rcu_read_lock(), old code still works */
> +	task = bpf_get_current_task_btf();
> +	real_parent = task->real_parent;
> +	bpf_printk("pid %u\n", real_parent->pid);

nit. Can bpf_printk be avoided here?

Others lgtm.